1. Field of the Invention
This invention relates to an antenna system to be used for satellite communications or broadcasting such as INMARSAT system, and more particularly to an antenna system incorporating a stabilization function against inclination of a moving platform.
2. Description of the Related Arts
Heretofore directional antennas have been used to perform maritime satellite communications.
Historically, the maritime communications were started in 1976 by using the American MARISAT system. It was handed over to the internationally organized INMARSAT system in 1982 and has been in operation since then. Antennas having the predetermined directivity are indispensable for such maritime satellite communications.
According to the technical requirements document for the standard-A ship earth station in the present INMARST system as of June 1987, the ship earth station should have G/T of -4 dBK at least. To meet this requirement, a parabolic reflector antenna should have a diameter of about 80 centimeters, for example.
Further, a radome is necessary to make the parabolic reflector antenna resistant to water and rough weathers. The radome should be about 1.2 meters in diameter for the parabolic reflector antenna having 80 centimeters diameter.
Some example of the conventional antenna systems will be described here with reference to the accompanying drawings.
FIGS. 18 and 19 show the configuration of a first conventional antenna system having a radome. The antenna system in these drawing figures is cited as the prior art in the co-pending Japanese Utility Model Application Hei 2-89713. FIG. 18 is a perspective view, and FIG. 19 is a side view.
A parabolic reflector antenna 10 having about 80-cm diameter is covered by a radome 12 made of material which can pass the radio waves whose wavelength is about 1.5 GHz necessary for the satellite communications. The radome 12 is usually made of FRP. The radome 12 has a maximum diameter of about 1.2 meters, and about 1.1 meter diameter at its bottom.
The antenna 10 is supported by a pedestal 14. A DC power source 16 and an power amplifier 18 are mounted on the base of the radome 12. The power amplifier 18 amplifies an output of a receiver 20 (or a receiver front end) mounted on the rear side of the antenna 10, and supplies it as a receiving output to an exterior unit. The DC power source 16 supplies DC power to the receiver 20 and power amplifier 18.
An access hutch 22 is made on the radome base and is about 40 centimeters in diameter. The access hutch 22 is opened and closed for inspection, maintenance and repair of the receiver 20, etc. The antenna system can be attended by exchanging units or connecting measuring instruments through the access hutch 22, for example.
To install such antenna system on a moving platform such as a ship, firstly the radome base is fastened on a support, which is then fixed on a ship deck by a bracket. The antenna system is also suspended by a wire rope to be supported more reliably. To facilitate the maintenance work via the access hutch 22, a platform for the work is usually mounted on the upper part of the support, e.g. about 75 centimeters below the radome base.
The pedestal 14 is fixed on the radome base by its foot. Therefore, the support should be in coaxial with the foot of the pedestal 14 via the radome base.
The antenna system on the moving platform should be steered to track a communication satellite to keep on receiving radio waves under optimum conditions from the satellite. For this purpose, the antenna should be stabilized against inclinations of the moving platform.
Inclinations are caused by rolling and/or pitching. For the stabilization, the antenna should be mechanically or electronically steered in the angular directions. A number of methods have been developed to steer the antenna to stabilize the antenna.
FIG. 20 shows an example of an antenna system in which three axes are mechanically steered.
As shown in FIG. 20, the antenna system includes a dish 24 supported on a ring 26 via an axis. The axis (i.e. dish axis) for the ring 26 to support the dish 24 is connected to a dish axis driving motor 28. Therefore, the dish 24 is set in motion by the dish axis driving motor 28.
The ring 26 is supported to an assembly body 30 via a ring axis, which is connected to a ring axis driving motor 32. The ring 26 is set in motion by the ring axis driving motor 32.
The assembly body 30 is angularly moved by an above-deck electronic assembly 34.
In this conventional example, three axes are mechanically steered. Specifically, the dish 24 is angularly moved about the X, Y and Az axes as shown by lines with arrows.
Therefore, the antenna stabilization is performed by operations of the motor 28, 30 and the assembly 34 when a signal is transmitted or received by the dish 24 via a low noise amplifier (LNA) 36 or 38 and a diplexer (DIP) 38.
In this conventional antenna system, all the three axes are mechanically steered, which complicates design of the system mechanism, making the system large and expensive.
To overcome such inconvenience, an antenna system is proposed, in which two axes are mechanically steered.
FIG. 21 shows the configuration of axes in an Az-El antenna mount.
This mount includes an Az (azimuth) axis which is perpendicular to a horizontal plane and an El (elevation) axis which is parallel to the horizontal plane and is turned by the Az axis.
The following literatures are available concerning the configuration of the Az-El mount: "Control Method of 2-Axis Az-El Antenna Mount", by Yuki et al., Electronic Communications Society, SANE83-53, pp 1-6), and "Development of a Compact Antenna System for INMARSAT Standard-B SES in Maritime Satellite Communications", by Shiokawa et al., Electronic Communications Society, SANE84-19, pp 17-24. These literatures disclose the configuration of apparatuses which were experimentally manufactured by the authors. In these apparatuses, the two axes, Az and El axes, are mechanically steered.
It is possible to stabilize the antenna by foregoing 2-axis configulation while tracking.
However, since the two axes are mechanically steered, a number of measures should be taken to overcome inconveniences related to a singular point.
Specifically, the singular points tends to appear in the direction of the zenith. When the antenna faces to that direction while it is being inclined, a tracking error will be caused. To cope with this singular point, the following measures are taken in the third example of the conventional antenna system: (1) The antenna and its support frame are made of light and rigid material to reduce a load applied to a motor for steering the antenna. A relatively high performance AC servo motor is used together with a high performance AC servo control circuit to steer the antenna. (2) Control software is improved to reduce tracking errors near the singular point.
However since these measures require special material and expensive circuits, the entire antenna system will be expensive. Even if the above-described measure are taken, there are still data that a tracking error of about 10.degree. is inevitable near the zenith.
The foregoing inconveniences can be solved by forming at least one of a plurality of the axes as an electronic axis. The electronic axis is realized by a phased array antenna.
FIG. 22 shows an antenna system including two electronical axes as well as a mechanical axis. Such antenna system is disclosed in "Phased Array Antenna for MARISAT Communications". Folkebolinder, Microwave Journal, 1978, 12, pp 39-42. This array antenna system includes a mechanical axis 42 as an Az axis and a plurality of antenna elements 46 mounted on two plates 44-1, 44-2 (i.e. array antenna) which are phase-shifted to serve as an El axis.
Specifically, the antenna elements 48 are arranged in matrix on the plates 44-1, 44-2. The antenna elements are individually connected to non-illustrated phase shifters whose amount of phase-shift changes in response to control signals. Therefore, when the amount of phase-shift is controlled for each row of the antenna elements on the planes shown by FIG. 22, the beam directivity in a direction parallel to the columns is controlled, thereby forming an El axis as an electronic axis. According to the foregoing literature, the beam directivity is changeable in the range of .+-.35.degree.. Such electronic El axis is realized by the array antenna and its phase shifters.
Another electronic axis can be realized by the foregoing configuration. Specifically, amount of phase-shift is controlled for each column of antenna elements 46 so that the beam directivity is changed in a direction parallel to the columns, thereby one more electronic axis is realized. Therefore, in the fourth example, the Az-El-El' mount with two electronical axes is realized.
The foregoing antenna systems are covered with radomes, and include a number of driving mechanism according to the axis configuration.
However, even when the electronic axis is used as described above, phase shifters should be provided for each antenna element 46, making the entire antenna system large, complicated and expensive. Therefore, application of the foregoing antenna systems is somewhat limited.
In the co-pending Japanese Utility Model Application Hei 2-89713, a compact and easy-to-maintain antenna system is proposed to improve the first example of the conventional antenna system. The proposal is made to use a compact radome. Although the radome base is reduced in size when an antenna is small, an access hutch is kept large. Specifically, the antenna and the pedestal is eccentically supported on the base with a space maintained above the base.